In various kinds of laboratory work, it is often necessary to transfer precise, small quantities of liquid from a first group of cavities to a second group of cavities. Such a need exists, for example, in microbiological and immunological laboratory work, as well as medical laboratory research.
This liquid transfer function is characteristically carried out with a pipetting device which includes a housing. The open ends of the tubes are placed into the first group of cavities and the pipetting device is coupled to a source of air at less than atmospheric pressure to thereby draw liquid into the tubes. The first group of cavities is then removed, and the tubes are placed into a second group of cavities. The vacuum pressure within the housing is then relieved to expel the liquid in the tubes into the second group of cavities.
The use of a pipetting concept to accomplish this liquid transfer is sound. Unfortunately, the devices developed heretofore for the purpose of carrying out this pipetting function possess several drawbacks. For example, the use of positive and negative air pressure to control the operation of the pipetting device, as exemplified by Lefkovitz U.S. Pat. No. 3,807,235 and Byrd U.S. Pat. No. 3,982,438, do not provide the accuracy required for laboratory work. In this regard, the volume of liquid transferred per cavity is typically in the microliter range and precise results are required.
In addition, the devices shown in the Lefkovitz and Byrd patents are capable of transferring only fixed volumes of liquid. For example, in order to alter liquid transfer volumes with the pipetting device shown in the Byrd patent, it is necessary to disassemble the device and replace one plate with another. This wastes time and increases the cost of the laboratory procedure.
The first and second group of cavities referred to hereinabove are characteristically provided in first and second trays, respectively, of standardized dimensions and having standard spacing between the cavities. As the ends of the tubes must be received in these cavities, the standardization of the trays controls the spacing between tubes. Typically, the center-to-center spacing of the cavities in the tray and hence of the tubes of the pipetting device is from seven to eight millimeters. Some liquid transfer operations require the transfer of 200 microliters of liquid per cavity. Because of the close center-to-center spacing of the tubes, the prior art pipetting devices do not provide adequate liquid transfer volumes.